Glutamine synthetase from Acetobacter diazotrophicus, an endophyte originally isolated from sugarcane, was studied as a step in the identification of mechanisms underlying the role of A. diazotrophicus as a major supplier of fixed nitrogen to its host plant. The enzyme was purified and partially characterized. It was also shown that the enzyme is regulated by adenylylation in response to the nitrogen source. Interestingly, there is no upregulation of the synthesis of the enzyme under diazotrophic conditions, which is in contrast to the situation in enterics, e.g. Klebsiella pneumoniae.